Plastic marking process



A ril 21, 1970 R. F'. TAMM ET AL PLASTIC MARKING PROCESS Filed July 24,1967 TO POWER SUPPLY INVEN TORS RICHARD F TA MM JAMES M. THRONE AT Y.

3,507,655 PLASTIC MARKING PROCESS Richard F. Tamm, Elmhurst, and JamesM. Throne, Country Club Hills, Ill., assignors to Continental CanCompany, Inc., New York, N.Y., a corporation of New York Filed July 24,1967, Ser. No. 655,387 Int. Cl. G03c 1/92 U.S. Cl. 96-451 8 ClaimsABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of theinvention The present invention relates to the marking of plasticsubstrates and more particularly relates to a means for marking aplastic material having fluorescent properties using optical radiation.

The prior art In the packaging of materials in plastic containers, it isfrequently desired to apply to the surface of the container somedesignation or code number which serves to indicate the source of theparticular lot of material being packaged, the time or date when theparticular material was packaged, or other information. Many times it isalso desirable that the code or marking be invisible, as for example,where it is desired to follow the manufacturing history of the containerWithout defacing the container.

Generally, the code marking is embossed on the plastic container surfaceby metal dies which is a time-consuming and expensive procedure.

SUMMARY OF THE INVENTION The present invention provides a means ofproducing on a plastic substrate having fluorescent properties a markinginvisible in ordinary light but which can be read when subjected to theproper selective light or energy Wave length wherein an opaque stencilhaving an aperture pattern corresponding to the desired marking isplaced between the substrate to be marked and a source of opticalradiation, the substrate being so positioned that the aperture patternof the stencil is in registration with the portion of the substrate onwhich it is desired to produce the invisible marking, whereupon thesource of optical radiation is impinged on the substrate through theapertures of the stencil, the source of optical radiation transmittingsufficient energy to the substrate surface to cause a change in thefluorescence of the substrate exposed to the source of opticalradiation.

DESCRIPTION OF THE DRAWING In the accompanying figure is illustrated aschematic representation of an apparatus for projecting a source ofoptical radiation through an opaque stencil to produce a markinginvisible in natural light but visible in ultraviolet light on thesurface of a fluorescent plastic substrate.

Referring to the figure, a plastic substrate 10 which exhibitsfluorescence in filtered ultraviolet light is transportable from supplyroll 11 past a marking station indicated United States Patent 03,507,655 Patented Apr. 21, 1970 "ice by the arrow 12 to be taken up ontake-up roll 13. The fluorescent plastic substrate 10 is sensitive tothe optical radiation from a source 14 which is directed thereon throughstencil 15 having aperture pattern 15A corresponding to the markingwhich is desired to be printed on the surface of the substrate 10.

The stencil 15 is placed adjacent to the moving substrate 10 in aposition at which the aperture pattern 15A is in registration with theportion of the substrate 10 at which it is desired to mark the substrateas it passes through the marking station 12.

The source of optical radiation 14 is illustrated diagrammatically asone or more lamps 16 with series current distribution connectionsthereof which are located in close proximity to the moving substrate 10.The lamps 16 are housed in an opaque base 17 having an opening 18 and areflector 19 to reflect light rays from which the optical radiation isprojected to the stencil 15. The lamps 16 are electrically connected toa power supply which furnishes high voltage direct current forenergizing the lamps.

The lamps 16 will be understood to be such as will radiate any suitableform of optical radiation which will furnish an amount of energy whichis effective to cause a change in the fluorescence of the fluorescentplastic substrate.

For example, the lamp 16 can be formed from a tube or envelope made offused quartz which contains an inert gas such as xenon or krypton underreduced pressure. Such lamps emit a high energy continuous spectrumlight when energized.

The wavelengths of the light emitted by the lamp 16 are both in theinfra-red and ultra-violet range and include as well, wavelengths ofsome or all the light in the intermediate or visible spectrum.Ultraviolet light having wavelengths ranging from 4000 angstroms, forexample, to a lower limit of 1800 angstroms, which is the limit for thetransmission of ultra-violet light through quartz, is particularlydesirable for the purpose. of the present invention and a light sourceof the type described is particularly efficient in producing anabundance of light between these wavelengths.

The source of optical radiation 14 may be operated continuously withexposure controlled by means of a suitable shutter or of theinstantaneous discharge or flash type which may be operatedintermittently as required for exposure.

PREFERRED EMBODIMENTS The areas of the plastic substrate marked byexposure to the optical radiation are invsible in ordinary light butexhibit a fluorescence different from the fluorescent color of theunexposed areas of the fluorescent substrate in accordance with thepattern of the apertures in the stencil when viewed in ultravioletlight. The visual impact of the marking results from the contrastdeveloped between the initial background and the changed fluorescentareas and can be readily identified. The change in fluorescence in theradiated area can be either significantly more or less fluorescence orfluorescence of a significantly different color or wave length.

There are many polymeric materials known to the art which exhibitfluorescence in filtered ultraviolet light. For example, when viewed inultraviolet light, synthetic resins such as phenol-formaldehyde resinshave an intense blue-violet fluorescence, thiourea-formaldehyde resinshave a bluish-white fluorescence, cellulose-acetate has a faintbluish-white fluorescence, cellulose-nitrate has a yellowbrownfluorescence, polyacrylic acid has an intense blue fluorescence,polybutadiene has a bright violet fluorescence, polystyrene has ablue-violet fluorescence, chlorinated rubber has a pale light bluefluorescence, polyvinylacetate has a white-blue fluorescence,polyvinylalcd hol has a white fluorescence, polyvinylchloride has abluegreen fluorescence, and epoxy resins exhibit a light blueflourescence.

The fluorescence of common plastics in filtered ultraviolet light may befound in the Handbook of Plastics by H. R. Simonds and C. Ellis vanOstrand, 1943, Table 107.

The amount of optical radiation transmitted through the stencil Will bedependent upon the voltage and energy from a suitable source. Generally,the amount of the optical energy emitted by the radiation sourcesufficient to cause a change in the fluorescence of the fluorescentplastic substrate may vary from about 1 to about 6 joules per squarecentimeter of substrate surface.

Various sources of optical radiation may be employed, such asxenon-filled flash tubes, argon-filled flash tubes, mercury-vapor flashtubes, or xenon, argon or mercuryvapor continuous radiation tubes.

To illustrate the manner in which the process of the present inventionmay be carried out, the following examples are given. It is to beunderstood, however, that the examples are for the purpose ofillustration and the invention is not to be regarded as limited to anyof the specific materials or conditions recited therein.

Example I To a tinplate surface was applied at a dry film weight of 3.5mg./in. a coating solution composed of 21.93% of an epoxy resincommercially available from the Shell Chemical Company under thetradename EPON 1007 represented by the formula:

7.32% of a mixture of allyl ethers of mono-, diand trimethylol phenolshaving the following structure:

CHzOH) wherein x is an integer of 1 to 3 commercially available from theGeneral Electric Company under the tradename GE 75108 Methylon; 0.30% ofa polyvinyl butyral resin having a molecular weight of about 50,000commercially available from Shawinigan Resin Company under the tradenameButavar B76, and 0.45% phosphoric acid, the above components dissolvedwith 70% Cellosolve acetate.

The coating solution was baked minutes to a cured film at 400 F. Whenexamined under ultraviolet illumination using a Raytech HandIlluminator, the coating had a light blue fluorescence at 3200 to 4000AU and a light violet fluorescence at 2800 AU.

An aluminum stencil having an aperture pattern having a numerical designwas placed between the epoxy resin substrate and a source of opticalradiation which emitted a high energy light of a continuous spectrum.The source of optical radiation was a flash emitted by an array of fourseries connected xenon-filled quartz flash tubes. The flash emitted hada duration of approximately 150 microseconds and was energized by a 980watt-second pulse of energy delivered by a 160 microfarad bank ofcapacitors charged to 3500 volts. The amount of radiant energytransmitted to the substrate by this flash was 3 jou es/cm. of substratesurface.

.4 The flash tubes were spaced inch apart and a polished metal reflectorwas mounted in contact with the back surface of the flash tubes toreflect the light rays onto the epoxy resin substrate. The coatedsurface was also spaced one inch from the plane of the flash tubes.

The coated plate was examined under daylight conditions and no markingcould be observed on the coated plate. However, when the coated platewas examined under ultraviolet illumination, the numerical marking wasobserved on the coating surface which stood out due to the contrast influorescence between the light blue fluorescence in the non-flashed areaand the relative non-fluorescence in the flashed area.

The clarity of the printed image was measured by determining theintensity of visible light which would just extinguish the image. Thisis defined as the voltage applied to a watt projector lamp placed 15inches distance at a 10 incident angle from the sample. A simplenomograph was used to convert actual voltage over a range from 20 tovolts to a corrected range of from 0 to 100 volts. The numerical markinghad an extinction voltage of 35.

The marked sample was then immersed in water at F. for 20 minutes. Aslight loss in clarity of the mark was observed, although the mark wasstill readable in ultraviolet light at 3200 to 4000 AU and below 2800AU.

Example II The procedure of Example I was repeated with the exceptionthat small amounts of organic fluorescent materials, were added to thecoating formulation. More intense markings were observed in ultravioletlight than with the unmodified coating formulation when the coatingsurface was irradiated in accordance with the procedure of Example I.The fluorescent materials added to the coating formulation of Example I,their concentrations, and the extinction voltages of the markings inultraviolet light are recorded in the table below.

TABLE Amount of Fluorescent material added to coating ExtinctionFluorescent material formulation (phr.) voltage 4-methyl-7-diethylaminocournarin 0. 5 68 4-methyl-7-diethylammo comnarin 0. 05 38 Sodiumfluorescein 0. 05 42 1 Parts per 100 parts resin solids.

Only slight losses in marking clarity were observed after immersion ofthe above irradiated samples in water at 160 F. for 20 minutes.

Example III Example IV An epoxy/vinyl chloride copolymer was prepared byreacting one mole of vinyl chloride with 0.248 mole of glycidylcrotonate at 80 C. using 0.2 mole percent of benzoyl peroxide catalyst.

At 28% conversion, a copolymer containing 28.6% glycidyl ester and 40.2%chlorine was obtained. The intrinsic viscosity of the copolymer was0.187 poise at 30 C. Forty grams of the copolymer were dissolved in amixture of 51 grams of toluene and 9 grams of isopropanol along with onegram of Versamide 125 which is a condensation product of a polymerizedfat and a polyamine of the type described in U.S. Patent 2,379,413.

The coating composition was applied to a tinplate surface and baked 8minutes at 325 F. Cured coated samples were marked in accordance withthe procedure of Example I over the range of 2.5 to 4.0 kilovolts. Theamount of energy transmitted to the substrate over this range was 1.5 to3.5 joules/cm? None of the samples exhibited a print visible in normallight but all exhibited a clearly readable coaded image underultraviolet light. At 3200-4000 AU, the coded image was anon-fluorescent marking on an overall fluorescent background. At 3.5kilovolts, .i.e., at 3.0 joules/cm? of substrate surface, imagevisibility under ultraviolet light was excellent, as evidenced byextinction voltage of 75 volts.

The markings on the samples were found to be extremely stable, as therewas no sognificant alteration of the coded image when the flashedsamples were exposed to water at 160 F. for 20 minutes or exposed toroom illumination, that is, a cool, white fluorescent lamp and stored inthe dark for over 6 months.

Example V A film, about 4 mils thick, which was cast from a copolymercontaining 86% vinyl chloride, 13% vinyl acetate, and 1% maleic acid,was irradiated at 4.0 'kilovolts, 1250 joules, following the procedureof Example I using a reflective metal surface to backup the film. Theamount of energy transmitted to the substrate was 3.5 joules/cmF. Anexamination of both sides of the films indicated that the marking couldbe read from either side of the film.

What is claimed is:

1. A process for producing a marking invisible in natural light butvisible in ultraviolet light on a plastic substrate exhibitingfluorescent properties in filtered ultraviolet light which comprises thesteps of positioning an opaque stencil having an aperture patterncorresponding to the desired marking between the substrate to be markedand a source of optical radiation so that the aperture pattern is inregistration with the portion of the substrate on which it is desired toproduce the marking, impinging the source of optical radiation on thesubstrate through the apertures of the stencil, the source of opticalradiation transmitting suflicient energy to the substrate surface tocause a change in the fluorescence of the substrate in the areas exposedto the source of optical radiation, the

change in fluorescence being discernible only in ultraviolet light.

2. The process of claim 1 wherein the plastic substrate is comprised ofan epoxy resin.

3. The process of claim 1 wherein the substrate is a copolymercontaining vinyl chloride.

4. The process of claim 1 wherein the substrate is a vinylchloride/vinyl acetate copolymer.

5. The process of claim 1 wherein the substrate is a vinyl choride/vinylacetate/maleic acid terpolymer.

6. The proces of Example I wherein the source of optical radiation is aflash lamp which emits a high-energy, continuous spectrum light.

7. The process of Example 1 wherein the source of optical radiation is axenon-filled quartz tube.

8. The process of claim 1 wherein the source of optical radiationtransmits to the substrate an amount of energy in the range of about 1to about 6 joules/cm. of substrate surface.

References Cited US. Cl. X.R.

